The present invention relates to an image processing device for processing a signal output from an image sensor and outputting the processed signal.
In digital cameras that use a charge-coupled device (CCD), a complementary metal oxide semiconductor (CMOS) image pickup device or the like as an image sensor, such as digital still cameras, digital camera-equipped mobile phones and digital video cameras, an image processing device performs image processing for an image signal read from the image sensor and outputs the results to a display device and a recording device. The image processing includes correction of the black level, correction of the white balance and the like.
FIG. 3 is a block diagram showing a configuration of a conventional camera. Referring to FIG. 3, an image signal read from an image sensor 912 line by line is converted to a digital signal by an AD converter (ADC) 913, and then supplied to a preprocessing circuit 926 and a signal processing circuit 928. The preprocessing circuit 926 generates a correction signal based on the input image signal and outputs the generated correction signal to the signal processing circuit 928. The preprocessing circuit 926 is allowed to output a correction signal generated based on an image of one frame only after receiving the entire image signal of the frame. The signal processing circuit 928 therefore corrects an image to be processed using a correction signal generated based on an image preceding the current image by one frame.
In the camera of FIG. 3 having the above configuration, proper correction will not be obtained when an object to be imaged moves largely, and this may degrade the quality of the processed image. There is also known a camera as follows, in which an improvement has been made in the above point.
FIG. 4 is a block diagram showing another configuration of a conventional camera (a roughly identical configuration to this is disclosed in FIG. 4 of Japanese Laid-Open Patent Publication No. 3-74980, for example). Referring to FIG. 4, an image signal read from an image sensor 912 line by line is converted to a digital signal by an AD converter 913, and then supplied to a preprocessing circuit 926 and also to a buffer memory 924. The preprocessing circuit 926 generates a correction signal based on the input image signal and outputs the generated correction signal to the signal processing circuit 928, as in the camera of FIG. 3. The signal processing circuit 928 starts reading the image signal stored in the buffer memory 924 after a lapse of the time of one frame from the start of storage of the image signal in the buffer memory 924.
In the camera of FIG. 4, the signal processing circuit 928 corrects an image to be processed using a correction signal generated based on this image. Hence a high-quality image can be obtained.
However, the use of a buffer memory for temporarily storing an image increases power consumption. In addition, a buffer memory, which is often provided as an external memory outside the image processing device, consumes large power. These pose a large problem for portable cameras that often use a battery as the power supply. In view of this, processing with a buffer memory should desirably be performed only when necessary.
No external memory is provided in some cases for cost reduction of the entire system. In recent years, reduction in the development cost of image processing devices has been strongly demanded. Under these circumstances, it is desired that one type of image processing device should be adaptable to both a system having an external memory and a system having no external memory.